A critical evaluation of the mass transfer and fouling behaviour in forward osmosis with integrated flow-reversal

Hurter, Marielle (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Forward osmosis (FO) is a membrane-based technology that can be operated at relatively low mechanical pressures and may be utilised in processes where water needs to be added or removed from process streams. Options for its potential application are diverse and it could, for example, be used in the regulation of water content in fruit juices, or in the augmentation of clean water to high-TDS cooling water circuits. Similar to reverse osmosis (RO) processes, scale formation by sparingly soluble salts can limit the maximum allowed recovery of water, while flux profiles, salt rejection characteristics and cross-flow velocity (CFV) play key roles in the overall behaviour of the system. However, FO systems are more amenable to the utilisation of osmotic backwashing than RO systems. Therefore, this study endeavoured to critically evaluate the mass transfer and fouling behaviour of FO membranes at different operating conditions, including the intermittent switching of the flow path (i.e. intermittently reversing the flux). To support this study, a bench scale FO setup was designed, constructed and commissioned. Subsequent laboratory work entailed: - Evaluate and assess the bench scale setup by comparing the theoretical and measured recovery, based on the measured water flux. - Evaluate the effects of changes in the CFV on the mass transfer of water and solutes over the membrane, while using a feed solution with TDS well below 100 mg·L-1. - Determine the effects of the operational configuration on the mass transfer over the membrane. - Investigate the process realities and limitations of intermittent flow path switching on reducing scale formation. Two operational modes were considered, viz. with the membrane active layer (1) facing the feed solution (AL-FS) or (2) facing the draw solution (AL-DS), with CFVs ranging from 13 cm.s-1 to 52 cm·s-1.Within this CFV range, the water fluxes attained in the AL-FS configuration were on average 40%lower than those in the AL-DS configuration. In the AL-FS configuration, the flux increased from 11.2 L·m-2·h-1 to 20 L·m-2·h-1 when the CFV was increased from 13 cm.s-1 to 37 cm·s-1. However, a further increase in CFV above 37 cm·s-1 did not result in higher fluxes and the limiting flux of 20 L·m-2·h-1 was reached. This is ascribed to the potential increase in dilutive internal concentration polarisation in the support layer of the membrane, thereby limiting the effective driving force (effective osmotic pressure difference) over the membrane. In the AL-DS configuration, this limiting flux was not reached within the defined CFV range. However, it was found that operation in the AL-DS configuration tended to a limiting flux of 20 L·m-2·h-1 when operating at draw solution concentrations above 50 000 mg·L-1 TDS. This is considered to be partly the result of an increased reverse solute flux (RSF) along with dilutive external concentration polarisation on the active layer side of the membrane. During operation with intermittent flow path switching when recovering water from a 1.9 super-saturated gypsum feed solution, ca 15 minutes were required to purge the flow channels of the respective residual solutions in the specific laboratory system under investigation. Operation at a CFV of ca 28 cm·s-1 then proved to enable the most rapid alleviation of internal concentration polarisation (ICP) in the AL-FS configuration (or mostly RSF in the AL-DS configuration). Under the most stable conditions in the AL-FS configuration, the operational flux dropped from 12 L·m-2·h-1 to ca 9 L·m-2·h-1 over a period of only 12 hours. In other words, flux declines of ca 38% were observed over a period of 12 hours when operating in the AL-DS configuration at 15-minute switch-intervals every two hours. This indicated the formation of gypsum scale in the support layer and highlighted the detrimental effects of the support layer in a scaling environment.

AFRIKAANSE OPSOMMING: Voorentoe osmose (FO) is ʼn membraan-gebaseerde tegnologie wat bedryf kan word teen relatiewe lae meganiese druk en kan gebruik word in prosesse waar water bygevoeg of verwyder moet word van die prosesstroom. Opsies vir potensiële toepassings is divers en kan, byvoorbeeld, gebruik word in die regulasie van waterinhoud in vrugtesappe, of in die aanvulling van skoon water by hoë TDS verkoeling water kringlope. Soortgelyk aan tru-osmose (RO) prosesse, kan skilfer formasie deur spaarsaam oplosbare soute die maksimum toegelate herwinning van water beperk, terwyl fluks profiele, sout verwerping karakteristieke en kruisvloei snelheid (CFV) sleutel rolle in die algehele gedrag van die stelsel speel. FO stelsels is egter meer inskiklik vir die gebruik van osmotiese terugspoeling as RO stelsels. Daarom het hierdie studie gepoog om die massa-oordrag en bevuiling gedrag van FO-membrane by verskillende bedryfskondisies, insluitend die afwisselende omruiling van die stroomlyn (i.e. afwisselende omkering van die fluks), krities te evalueer. Om hierdie studie te ondersteun is ʼn banktoetsskaal FO-opstel ontwerp, opgerig en in bedryf gestel. Opvolgende laboratorium werk het behels: - Evalueer en assesseer die banktoetsskaal deur die teoretiese en gemete herwinning, gebaseer op die gemete waterfluks, te evalueer. - Evalueer die effek van veranderinge in die CFV op die massa-oordrag van water en opgeloste stowwe oor die membraan, terwyl ʼn voeroplossing met TDS ver onder 100 mg.L-1, gebruik word. - Bepaal die effek van die operasionele konfigurasie op die massa-oordrag oor die membraan. - Ondersoek die proses realiteite en beperkinge van afwisselende stroomlyn omruiling op die vermindering van skilfer formasie. Twee operasionele metodes is oorweeg, viz. met die membraan aktiewe laag (1) gerig na die voeroplossing (AL-FS) of (2) gerig na die trekoplossing (AL-DS), met CFVs binne bestek van 13 cm.s-1 tot 52 cm.s-1. Binne hierdie CFV-bestek, was die waterflukse bereik op gemiddeld 40% laer in die AL-FS-konfigurasie as dié in die AL-DS-konfigurasie. In die AL-FS-konfigurasie, het die fluks vermeerder van 11.2 L.m-2.h-1 tot 20 L.m-2.h-1 wanneer die CFV verhoog is van 13 cm.s-1 tot 27 cm.s-1. ʼn Verdere verhoging in CFV bo 37 cm.s-1 het nie hoër flukse tot gevolg gehad nie en die beperkende fluks van 20 L.m-2.h-1 is bereik. Dit word toegeskryf aan die potensiële verhoging in verwaterde interne konsentrasie polarisasie in die ondersteuningslaag van die membraan, wat sodoende die effektiewe dryfkrag (effektiewe osmotiese drukverskil) oor die membraan beperk. In die AL-DS-konfigurasie, was hierdie beperkende fluks nie bereik binne die gedefinieerde CFV-bestek nie. Dit is egter bevind dat bedryf in die AL-DS-konfigurasie ʼn neiging tot ʼn beperkende fluks van 20 L.m-2.h-1 gehad het as trekoplossingkonsentrasies bo 50 000 mg.L-1 TDS was. Dit word beskou om deels die resultaat van ʼn verhoogde omgekeerde opgeloste stof fluks (RSF) saam met verwaterde eksterne konsentrasie polarisasie op die aktiewe laag kant van die membraan te wees. Gedurende bedryf met afwisselende stroomlynomruiling toe water herwin is van ʼn 1.9 super-versadigde gipsvoeroplossing, is ca 15 minute benodig om die vloeikanale van die onderskeidelike oorblywende oplossings in die laboratoriumstelsel spesifiek tot dié ondersoek, te suiwer. Bedryf by ʼn CFV van ca 28 cm.s-1 is toe bewys om die spoedigste vermindering van interne konsentrasie polarisasie (ICP) in die AL-FS-konfigurasie (of meestal RSF in die AL-DS-konfigurasie), in staat te stel. Onder die mees stabiele kondisies in die AL-FS-konfigurasie, het die operasionele fluks geval van 12 L.m-2.h-1 tot ca 9 L.m-2.h-1 oor ʼn periode van slegs 12 ure. Met ander woorde, fluks afnames van ca 38% is waargeneem oor ʼn periode van 12 ure by bedryf in die AL-DS-konfigurasie met 15 minute omruil-intervalle elke twee ure. Dit dui formasie van gipsskilfer in die ondersteuningslaag aan en beklemtoon die nadelige effek van die ondersteuningslaag in ʼn verskalingsomgewing.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106148
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